Thermotropic liquid crystalline polymers are generally wholly aromatic condensation polymers that have relatively rigid and linear polymer chains so that they melt to form a liquid crystalline phase. Such polymers are commercially generated using bulk polymerization methodologies, such as melt acidolysis, which generally require the use of temperatures above the polymer melting point (260° C. to 380° C.) for extended periods of time. While wholly-aromatic polyesters display exceedingly high thermal stability, long-term heating at these high temperatures tends to lead to some degree of decomposition, which results in poorer color, increased volatiles, reduced product yield, and in extreme cases, compromised performance. Therefore, methods for reducing the temperature and overall time of exposure of these species are desired in the art.
One successful route for lowering the heat history of liquid crystal polymers (LCP's) involves generating a low molecular weight “prepolymer” species by standard routes, which is then subsequently heated at temperatures below the melting point of the polymer. This is referred to as “solid-state polymerization” and is typically used to provide higher molecular weight polymers suitable for injection molding, fiber spinning, and other end-use applications. Though the exact reasons are unknown, liquid crystalline polymers produced via solid-state polymerization tend to display enhanced thermal properties, such as higher heat distortion temperatures (HDT) and improved blister/off-gassing performance—both features being beneficial to commercial end-use applications. While advantageous from a property standpoint, standard solid-state polymerization methods are problematic. For instance, one such method involves heating the prepolymer in a tumble blended reactor. Unfortunately, however, this process requires long times to achieve high molecular weights owing to poor heat distribution/transfer from the reactor walls to the pellets. Thus, producing even medium molecular weight polymers in such systems can require solid-state cycle times higher than melt-polymerization production rates. This bottleneck can reduce the capacity of a plant to generate polymers with maximum thermal properties. Stated alternatively, the long cycle times required do not generally keep pace with the melt-polymerization production of the prepolymer. Additionally, solid-state polymerization times—despite the lower temperatures—add to the overall heat history of the materials, which is undesirable.
As such, a need currently exists for a method of improving the rate of the solid-state polymerization of a liquid crystalline polymer.